Good news: messaged LATAM on X, and they were able to submit the upgrade request, even though the website does not allow it.
Bad news: just checked the upgrade list and (possibly because of the current status match promo) it seems clear you really need to be Black Signature to have a realistic shot at an upgrade...
One thing I hadn't realized: if you have a LATAM credit card, then you receive higher upgrade priority than all the others in your same elite tier who don't have one.